Ignition systems of the above-described type are known, for example, from the German Auslegeschrift No. 2 549 586, corresponding to U.S. Pat. application Ser. No. 734,745. In the ignition system described there, the control signal, i.e., the ignition output switch closure signal, is increased abruptly at a given engine speed. Each ignition process requires a certain amount of energy to be stored in the ignition coil of the system which in turn implies that the time during which current flows through the coil should remain constant for all engine speeds. However, the control signal from an rpm-dependent sensor changes in the sense of decreasing the time of current flow for increasing rpm. In the known system, an attempt is made to compensate for this effect by the abrupt increase of the sensor signal beginning with a certain engine speed, thereby changing the trigger threshold for the sensor voltage.
Another system known from the German laid-open application No. 2 701 968, corresponding to U.S. Pat. application Ser. No. 869,627, continuously increases the ignition closure signal so as to attempt to maintain the ignition closure time constant for varying engine speeds.
The aforementioned known systems are not generally suitable for all sensors because some sensors, for example inductive sensors, generate signals which include negative and positive half-waves. In such a case the triggering thresholds of the subsequent threshold recognition circuits are so constructed as to be set in a first condition when the positive threshold is exceeded and then to be reset or placed in a second condition when the negative threshold is exceeded. However, some other sensors, for example Hall-effect sensors, generate unipolar output signals. Such signals are not suitable for addressing a threshold recognition circuit which is intended for a bipolar transducer because the second threshold is never activated.
Still another disadvantage of the known apparatus is that the signal generated for low engine speeds is far too large for the prevailing requirements and results in high power losses in the ignition coil.